The Earth’s Great Oxidation Event may have ended in a big dip

A sudden increase in oxygen levels around 2.4 billion years ago paved the way …

The rise of oxygen, which altered the planet's atmosphere and enabled multicellular life, may not have come as two large bursts, as has been widely held, but rather as several "whiffs." That is the provocative conclusion of a study of chromium isotopes in ancient sedimentary rocks published in Nature. A team of geochemists led by Robert Frei of the University of Copenhagen, Denmark, found that oxygen took a few twists and turns before reaching its present level.

With few deviations, the consensus among scientists has been that the first significant rise of oxygen occurred about 2.4 billion years ago. Previously, the concentration of oxygen in the atmosphere had been extremely low—much lower than one percent of today’s levels. That all changed with the advent of the Great Oxidation Event, or GOE, as the first spike in oxygen is known. A second large surge, which happened around 750 million years ago, made it possible for animal life to take hold.

The question of when and how oxygen levels first increased is still open to debate. Researchers have primarily tracked the event via banded iron formations (BIFs), sediments made up of alternating thin layers of iron oxides and iron-depleted silica that covered the ancient seafloor. BIFs act as a record of Earth’s early ocean and atmospheric chemical composition, and scientists have explored their chemistry through the use of metal isotopes in an effort to reconstruct a timeline of the period.

The early ocean was iron-rich but oxygen-poor. Iron entered the ocean from both the surface, through the weathering of continental crust, and from below the seafloor, through deep-sea hydrothermal vents. In the absence of oxygen, iron (Fe) existed in the ocean predominantly as a dissolved ion with two missing electrons (Fe2+). When oxygen entered the picture, however, it converted this to magnetite (Fe3O4) or hematite (Fe2O3), Fe-rich minerals that immediately precipitated out of solution and sank to the bottom of the ocean. Over time, these minerals accumulated in layers, which, when overlaid with Fe-poor silica particles, created the iron formations.

The oxygen produced by the ocean's first photosynthesizers is believed to have been entirely used up by this process. Eventually, however most of the dissolved iron was consumed, after which the gas was able to escape the ocean and flood the atmosphere. While this neatly explains why the GOE occurred about 2.4 billion years ago, it does not explain the sudden reappearance of BIFs in sediments deposited half a billion years later. For that to happen, oxygen levels would had to have dropped to near their pre-GOE levels somewhere in the intervening time.

Frei and his colleagues suggests that this drop may have indeed occurred. Like their colleagues, they studied the BIFs, but focused on a different metal: chromium (Cr). Chromium is oxygen-reactive and was deposited alongside iron in the BIFs. The researchers decided to use it as a tracer for the content of oxygen in the atmosphere.

Under anoxic conditions, Cr exists in a reduced state (Cr+3) in the continental crust. As oxygen begins to increase, manganese (Mn) becomes oxidized (i.e. it loses its electrons), forming manganese oxide (MnO2), which in turn reacts with Cr. That oxidizes it, producing Cr+6.

In its oxidized state, Cr becomes more motile, and is washed out of the soil and into the ocean. There, Cr reacts with Fe and returns to its reduced state, depositing within iron oxide layers. The reaction is so efficient that all of the Cr is removed from seawater.

During the reaction with MnO2, the heavy isotope of Cr, chromium-53, is preferentially oxidized relative to the light isotope, chromium-52. Geochemists would describe Cr as being more “enriched,” or positively fractioned, than chromium-53. The presence of positively fractioned Cr isotopes in an iron formation would therefore imply that oxygen levels were high at the time that it was deposited. Frei analyzed these isotopic signatures to estimate the concentration of oxygen that was present in the atmosphere at the time.

Their findings suggest that oxygen levels first increased 2.8 to 2.6 billion years ago, or at least 200 million years before previous estimates. Unexpectedly, they fell to pre-GOE levels approximately 500 million years later. That was followed by a prolonged period during which hydrogen sulfide accumulated in large quantities in the deep sea, creating oxygen-poor oceans, while atmospheric oxygen levels fluctuated. Then, almost 750 million years ago, there was another sharp increase. This likely corresponded with the evolution of the first animals.

If accurate—much more research using Cr and other metal isotopes will be needed to verify these findings—these results provide a clear picture of oxygen’s rise and fall during Earth’s early history and help explain when multicellular organisms first appeared. Yet some crucial questions remain unanswered. Were the levels of oxygen sufficiently high enough to produce the quantity of manganese oxide required to oxidize chromium? Is it fair to assume that the variations in chromium isotopes scale linearly to atmospheric oxygen levels, which the authors do?

There is also a good chance that chromium ions may have reacted with other compounds beside manganese oxide, further complicating the story. For now, though, Frei’s study stands as one of the first to offer a comprehensive explanation for how banded iron formations made an unprecedented comeback almost 500 million years after the GOE.